1. Field of the Invention
The present invention relates to a method and a drawing apparatus with an exposure unit that includes a plurality of modulation elements arranged in a matrix, for producing patterns on a surface of a workpiece.
2. Description of the Related Art
The above drawing apparatus is generally used to produce minute and fine patterns, characters, symbols, and so on, on suitable workpieces by an optical method. An example of a use of the drawing apparatus is for a circuit pattern drawn on a workpiece when a printed circuit board is manufactured by using photolithography. The workpiece may be either a photosensitive film for producing a photomask or a photoresist layer formed on a suitable substrate.
In recent years, the entire circuit board production process, from the circuit-board pattern designing process to the image-producing process, has been integrated and systematized into an integrated system in which a drawing apparatus plays a part. The integrated system is also provided with a CAD (Computer Aided Design) station, which is used for designing circuit patterns, and a CAM (Computer Aided Manufacturing) station, which processes vector data of the circuit patterns generated by the CAD station and produces work instructions. Vector data produced by the CAD station or processed by the CAM station are transferred to the drawing apparatus where the data are rasterized (vector-to-raster data conversion) and are then stored in a memory as bitmap data.
For example, an exposure unit having DMD (Digital Micro-mirror Device) array or LCD (Liquid Crystal Display) array is known; a DMD is a device with inclinable mirrors supported above each of memory cell that are arranged two-dimensionally in the silicone back plane. As is generally known, micro-mirror elements are arranged in a matrix on a reflective plane of a DMD and the light reflecting or inclined direction of each micro-mirror element is independently controlled. Therefore, light beams substantially uniformly made incident onto the whole reflecting plane of the DMD are individually reflected by each micro-mirror element and split into a plurality of beams, so that each micro-mirror element functions as a modulation element. In the LCD arrays, liquid crystal is intervened between a pair of transparent plate members, and multiple pairs of minute transparent electrode elements are complementary arranged in a matrix on each transparent plate. Transmissivity of the liquid crystal between each pair of electrode members is controlled by a voltage applied to each pair of electrode members, thereby each piece of the liquid crystal disposed between a pair of electrode members functions as a modulation element.
The drawing apparatus is provided with a light source which has suitable photosensitive characteristics for the workpiece, such as a supper high-pressured mercury-vapor lamp, a xenon lamp, a flash tube, LED (Light Emitting Diode), laser, and so on, and an exposure unit has an imaging optical system or a focusing lens system. Light beams from the light source are lead to the exposure unit through an illumination optical system or illuminating lens system. Each of the modulation units in the exposure unit modulates the incident light beams in accordance with the raster-data of the designed circuit pattern, thereby the workpiece is exposed and the circuit pattern is optically produced on the workpiece. The size of a pixel for the circuit patterns projected on the workpiece relates to the size of the modulation element. For example, when the magnification of the above imaging optical system is “1”, the pixel size for the projected circuit pattern and the modulation element size are substantially the same.
In general, the area of the circuit pattern to be produced on the workpiece is considerably larger than the area which can be exposed by an exposure unit or multiple units. Therefore, it is necessary to scan the workpiece with an exposure unit to produce the whole circuit pattern on the workpiece. Namely, to obtain the entire circuit pattern, it is necessary to produce each part of the circuit pattern in turn while moving the exposure unit relatively to the workpiece. Conventionally, a drawing table that is movable in a predetermined scanning direction, for example, is provided with the drawing apparatus. An exposure unit is fixed above the path of the drawing table and a workpiece is disposed at a predetermined position on the drawing table. The drawing table is intermittently moved along the scanning direction and only while the drawing table is at rest is each part of the circuit pattern produced in turn on a corresponding part of the workpiece. Hence the entire is circuit pattern is obtained as an assembly of consecutive parts of the circuit pattern. The above image-producing method is called the Step & Repeat method.
Further, an exposure unit with a laser beam scanning optical system, for example, is also known as another type of exposure unit. A drawing apparatus with an exposure unit of this type modulates laser beams, by using image data (raster-data), along a scanning line while scanning the workpiece with laser beams deflected to a direction that traverses the path of the workpiece translation, so that a required circuit pattern is provided.
The resolution of a circuit pattern produced by the above-discussed types of drawing apparatus is defined by the pixel size or a dot size that is predetermined for each drawing apparatus. Namely, for the drawing apparatus with modulation elements arranged in a matrix, the resolution of a circuit pattern is defined by the size of each modulation element or the pixel size. Also, in the drawing apparatus including the laser beam scanner, the size of the pixels is determined by the diameter of the scanning laser beam.
Accordingly, conventionally, when a circuit pattern is designed on a CAD or CAM station, it is necessary to coincide the pixel size of the circuit pattern to be designed with the pixel size determined by a drawing apparatus used. Thus, flexibility of circuit pattern design is restricted by the drawing apparatus. In other words, various types of drawing apparatuses must be prepared and used in the aforesaid circuit pattern drawing system, before the flexibility of circuit pattern design can be increased in the CAD or CAM station.
Further, in actual drawing processes, the positions of a circuit pattern to be drawn on individual workpieces differ, and also workpieces themselves can slightly expand or contract and become deformed. Therefore, conventionally, when a circuit pattern is required to be drawn after an accidental rotation of a workpiece or if a rotation of the pattern is required for some reason, the workpiece and an optical system are relatively rotated. Further, when a translation or scaling (scale-up or down) is required, it is achieved by using exclusive hardware attached to the apparatus. Thereby, the number of elements is increased, so that complex machine control is required and this may induce failure. Furthermore, when a straight line, in an original pattern, is transformed to an inclined line or a curved line by a coordinate transformation, a jagged edge which relates to the pixel size may be generated, so that a smooth line cannot be obtained.